The Colorado Department of Public Health and Environment sent letters to 8,000 patients of dentist Stephen Stein, urging them to seek tests for the diseases after learning of "unsafe injection practices" at two Denver-area clinics he owned between September 1999 and June 2011.

Investigators found that Stein reused needles and syringes in several patients' intravenous lines at his oral surgery and dental implant clinics, in violation of standard medical protocol, the department said in a statement.

"This practice has been shown to transmit infections," the statement said. It added that there had been no confirmed cases of anyone contracting the viral infections through Stein's clinics.

In the letters sent to Stein's former patients, the health department urged them to be tested for HIV, hepatitis B and hepatitis C, said Jan Stapleman, a department spokeswoman.

Stein's records were incomplete, so it is likely more people were possibly exposed than those already identified, she said.

A hot line established for Stein's patients to call was "very busy" all day on Friday, Stapleman said.

Stein's lawyer, Victoria Lovato, said her client "is cooperating with the state's investigation."

Authorities said any patient who underwent any type of injection at the clinics, including sedation, might be at risk. They cautioned that if any patients of Stein tested positive for any of the viruses, there was no way to determine how they contracted the disease.

Stein's license to practice dentistry in Colorado was suspended for an unrelated matter, said Cory Everett-Lozano, spokeswoman for the state Department of Regulatory Agencies, which oversees medical licenses.

Until that probe is resolved, the reasons for Stein's current suspension are confidential, she said.

Lynn Kimbrough, a spokeswoman for the U.S. Attorney's Office in Denver, said Stein was already the target of a criminal probe for possible prescription fraud before the allegations emerged about reusing syringes. She said no criminal charges had so far been filed.